1. Field of the Invention
The present invention relates to a device for purifying sewage including organic material by employing aerobes and, more particularly, to an improvement of such a device for controlling the thickness of a bed of the aerobes or activated sludge so as to avoid plugging of the bed.
2. Description of the Prior Art
In the conventional sewage purifying devices of the aforementioned type, there has been a problem that the bed of aerobes or activated sludge which is adapted to be traversed by sewage dissolved with oxygen becomes plugged after a period of operation due to a substantial increase of aerobes which are usually carried by a perforated frame structure of the bed. If the plugging of the bed occurs, resistance to sewage flow increases, causing poor circulation of sewage in a vessel of the purifying device. Furthermore, if the layer of aerobes becomes thicker, bacteria in an internal portion become anaerobic and thus the purification efficiency lowers.
A conventional method to remove the plugging of the bed is to wash the bed by water by flowing the same in a reversed direction, generally vertically upwardly through the bed. However, this method is uneconomical since it requires purification of the water which is used for washing. An alternative approach to the problem of the plugging of the bed has been to blow air upwardly through the bed. In the conventional approach, however, an air injection means is fixedly arranged below the bed to cover the entire bottom area of the bed and is adapted to supply air simultaneously to the entire body of the bed, whereby it requires a blower of a relatively large capacity which is operated only a few minutes with a relatively long interval, thus with a very low rate of operation.